Optically induced birefringence and surface mass transport in SU-8 photoresist doped with azobenzene chromophores

Summary form only given. Photoinduced orientation of azobenzene chromophores in polymeric host materials has been broadly explored for holography and related diffractive optics applications [1, 2]. Illumination of the chromophore with polarized light rotates the trans isomer perpendicular to the polarization, resulting in spatially modulated birefringence. In the dark, photo-oriented chromophores thermally relax to a random orientation resulting in a decay of birefringence over a wide range of time scales depending on the chromophore molecular architecture and glass transition temperature. A related phenomenon is that illumination with spatially modulated light (e.g. from two interfering laser beams) generates strong photo mechanical anisotropy in azo-polymer films, resulting in surface mass transport and the formation of periodic surface relief gratings [3, 4].Here we report photoinduced birefringence using the photoresist SU-8 doped with the azobenzene chromophore Disperse Red 1 (SU8-DR1). Although SU-8 is widely used in lithography, its use as a host polymer for incorporation of birefringent and orientable azo chromophores is relatively unexplored. Up to 10 wt% DR1 could be blended with SU-8 with no added solvent. SU8-DR1 solution was spin-coated onto glass substrates, prebaked, cured with UV light, and post-baked, resulting in 2 micron thick films of high optical clarity. Birefringence was induced with a 1.5 mW 490 nm laser and probed at 633 nm, with typical values of Δn = 0.005. Following removal of the 490 nm writing beam, the birefringence rapidly decayed to 50% of its initial value, then slowly decayed to zero over a period of 60 days. Illumination of SU8-DR1 with intensity and/or polarization gratings also resulted in sinusoidal surface relief patterns with periodicity 14 micron as controlled by the interbeam crossing angle of the 490 nm writing beams; the surface modulation was +/40 nm as measured by atomic force microscopy (figure 1).SU-8 as a host polymer for photoinduced birefringence and surface relief grating applications is particularly interesting because it is UV and thermally cross-linkable, resulting in a hardened polymer film of high glass transition temperature [5]. This provides considerable flexibility in film processing for the creation of stable birefringence gratings for use in polarization holography and diffractive optics applications.